Thermal Field Emission (TFE) cathodes are commonly used in high-resolution electron probe tools such as scanning electron microscopes (SEM), critical dimension scanning electron microscopes (CD-SEM), and similar devices. A conventional TFE typically employs a needle electrode of tungsten single crystal and a coating layer of zirconium and oxygen formed on the tungsten single crystal tip. A voltage applied to the needle creates a strong electric field proximate the tip of the needle. A combination of the heating and strong electric field causes field emission of electrons from the tip of the needle.
FIG. 1 is a schematic diagram of a conventional thermal field emission cathode 100. As shown in FIG. 1, the TFE cathode 100 includes an emitter tip 102 for emitting an electron beam and a source or a reservoir 104, which is bonded to the shank of the emitter tip 102. The emitter tip 102 may be a single crystal tungsten (W) tip. The reservoir 104 supplies an active metal, e.g., zirconium or similar active metal (Titanium, Hafnium, etc) in the form of a metal compound (oxide, nitride, etc). The TFE cathode 100 also includes a heater 106, which is made from a refractory metal wire and shaped in to a “V” like structure. Typically, the heater 106 is in the form of a tungsten filament. The heater 106 is physically and electrically connected to a conductive pins 108 bonded to an insulating structure. The emitter tip 102 is bonded to the apex of the heater 106. During normal operation, the emitter tip 102 is heated to incandescent temperature of about 1800K. Heat is generated by passing electrical current through the filament 106, which heats the emitter tip 102 by thermal conduction. Electron emission is enhanced by activating the emitter tip 102 with Zirconium or similar active metal (Titanium, Hafnium, etc) in the form of a metal compound (oxide, nitride, etc) from the reservoir 104. By way of example, the emitter tip 102 may be covered by a ZrO coating layer. The end of life of the cathode occurs when the reservoir 104 is depleted. The TFE Cathode requires an operating vacuum of 10−9 to 10−11 torr range.
Because the metal carbides have high melting temperatures it may be problematic to weld the emitter tip 102 to the filament 106. Other methods for holding the tip 102 include a Vogel mount where the tip 102 is sandwiched between two blocks, typically carbon blocks, but not limited to carbon blocks. The tip 102 may be flashed by passing current through the carbon blocks to heat the tip. Another method is an indirect heating method where a metal carbide rod can be heated using electron beam bombardment. Here a filament that surrounds the metal carbide rod is heated and thermionic electrons from the filament may be accelerated to the metal carbide rod by applying an appropriate voltage between the rod and the filament.
Electron emission from metal carbides has been studied for several years. Metal carbide cathodes have electron emission properties that make them attractive candidates for stable emission sources in moderate to poor vacuum applications. Single crystal hafnium carbides (HfC) and other metal carbides (TiC, NbC, etc) have been investigated as alternative to W for the use as an electron emitter. The use of HfC <100> provides a highly refractive and relatively low work function (3.4 eV) emitting surface that has a low evaporation rate, is resistant to ion bombardment and sputtering, has a high melting point (˜4200K) and a very low surface mobility. These properties enable an HfC emitter source to operate at high current densities and also to have a long lifetime in poor vacuum conditions.
A simple method for preparing refractory carbide suitable for use as emitter tips from metal wires such as Ta, Zr, Hf, Nb and Ti utilizing a solid-vapor reaction is disclosed in “A New Preparation Method of Refractory Carbides and Their Thermionic Emission Properties” by Kerji Yada, in Journal of Electron Microscopy, Vol. 31, No. 4, pp 349-359, 1982, the entire contents of which are incorporated herein by reference.
It is desirable to sharpen metal carbide for use as emitters. Sharpened metal carbide tips also have applications in electron microscopy and related fields, such as Atomic Force Microscopes (AFM), and ion beam systems. A conventional method for making a metal carbide emitter sharp tip involves an electro-chemically etching process. In this process a metal carbide tip is immersed in an etching bath, typically NaOH. This technique is sometimes referred to as the “drop off” technique because the etching only takes place at surface of the tip. As the material thins the part that is below the surface eventually breaks and drops off.
Another conventional method for sharpening an emitter tip is to subject a surface of the metal carbide tip to a flux of inert ions, e.g., Ar+ ions from a flood Ar+ source, while rotating the tip about a longitudinal axis. Ar+ bombardment of the surface of the tip breaks and drops off portions of the tip. Another method for sharpening an emitter tip is to expose the tip to C2H4 to grow a carbon tip on the end of the metal carbide.
Unfortunately, such processes for sharpening the tip typically require removal of the tip from vacuum or disassembly of the dip from the electron probe tool or both for sharpening.
It is within this context that embodiments of the present invention arise.